This is a great question and there is no easy answer. I was also on the same page as to having a really amazing design for our app/start (Mugasha.com). I am not a designer myself, I am developer/UI/UX guy although, I have an appreciation for good design ethics. There are two ways to go about it, One is what 280North did and hire these mini-studios. They are great, focused and quite talented. Make sure its a mini one, big ones are not very focused. The second is the path I choose and found a designer in our local are through networking and events and made him a partner. This is tough, I am giving away a quarter of my company to a designer who I trust. I am also making him responsible and take ownership of the design and UI. Its a collaborative effort in the end but the best thing to do would be to hire someone who is good and who you can trust.